US Senate Democratic leader Harry Reid issued a strong defence yesterday of Senator Dianne Feinstein, who heads the Senate Intelligence Committee, in a dispute in which the committee and the CIA each allege they were spied upon by the other.

Reid, in a letter to Attorney General Eric Holder, called the Central Intelligence Agency's search of the Senate panel's computer network a "serious breach" and "indefensible".

In a Senate floor speech last week, Feinstein accused the agency of searching computers used by committee staffers examining CIA documents for information on counterterrorism operations and the use of torture methods like waterboarding.

Reid said the agency's actions cause him "great concern" as leader of the US Senate.

"The CIA has not only interfered with the lawful congressional oversight of its activities, but has also seemingly attempted to intimidate its overseers by subjecting them to criminal investigation," Reid said.

"These developments strike at the heart of the constitutional separation of powers between the legislative and executive branches," he added.

The CIA also sent a "crimes report" to the Justice Department about committee staff trying to get classified information.

Saying the dispute raised concerns about potential violations of the constitution, Reid said he would have Senate authorities begin a forensic examination of computers assigned for exclusive use by Senate intelligence staff.

"The CIA's decision to access the resources and work product of the legislative branch without permission is absolutely indefensible, regardless of the context," he said.

In the course of the panel's investigation, committee staff got documents from a review ordered by ex-CIA director Leon Panetta and apparently took those documents to the Capitol.

Reid said he had "instructed the Senate sergeant-at-arms to initiate a forensic examination of the computers and computer network assigned for exclusive [committee] use, in order to determine how the 'Panetta review' entered into the [committee] network".